1. Field of the Invention
Bulk elastomer stock used for the manufacture of rubber goods is conventionally prepared by coagulating a latex emulsion or emulsified latex masterbatch formulation with alum, acid, or other suitable agent. The resultant coagulum is washed free of the coagulation liquids and subsequently dried. Despite the fact that the wet curds may be in a particulate form, they will invariably congeal into a solidified mass upon drying. Consequently, an energy-intensive and time-consuming, high shear milling or mixing step is required for further formulation and processing.
The instant invention relates to a process for treating the wet coagulum so as to prevent congealing and to promote the production of free-flowing powdered elastomer compositions.
2. Description of the Prior Art
Previous attempts to produce powdered elastomer compositions have entailed coagulation of the latex or masterbatch formulation in the presence of a colloidally dispersed cereal grain polysaccharide such as pregelatinized starch, pregelatinized flour, aminated starches, and starch xanthate. In U.S. Pat. No. 3,673,136 and 3,714,087, Buchanan et al. teach that a free-flowing, fine powder can be produced if the polysaccharide elastomer coprecipitate is ground in a conventional comminuting machine after drying. Addition of powdered curatives prevent reagglomeration upon storage. Curatives and other additives can be mixed with the powdered coprecipitates in a simple powder blender which requires much less time and energy than the high shear milling or mixing equipment required by the conventional process. However, widespread commercialization of this technique has been inhibited by the expense and precise control required by the comminuting process, as well as by the mediocre properties of the vulcanized rubber products formed from the powdered compositions.
In U.S. Pat. No. 3,830,762, Abbott teaches that by repeatedly washing the polysaccharide-elastomer coprecipitate with a water-miscible solvent until the moisture content is between 0 and 10%, the necessity for comminution is eliminated. An improvement of this method is taught by Abbott in U.S. Pat. No. 3,941,767, wherein the precipitate is subjected to a second encasement prior to the washing step, thereby further reducing tackiness and enabling minimal incorporation of the polysaccharide material. While the best results were obtained with starch xanthate as the polysaccharide material, industrial acceptance of the process has been hindered by the toxicity and flammability of this agent. Moreover, the numerous processing steps required by the repeated encapsulations and solvent washings were found to be undesirable for a commercial procedure.